For example, Patent document 1 describes a first example of a vehicle with two parallel wheels of such kind in related art. Patent document 1 also describes a wheel used as a component of transfer equipment. A step clearing mechanism described in Patent document 1 is characterized by that “in the step clearing mechanism wherein a leg member having a disk shape or irregular shape periphery of a diameter greater than that of wheels used in the caster touches a step and rotates before the wheels themselves touch the step so that it causes the wheels to rise from the ground and run on the step, wherein the one leg member is arranged at the middle of the two wheels and integrated within the same support frame as that of the wheels, the pivot of the leg member being also supported within the same support frame”.
In accordance with the step clearing wheels having such structure of Patent document 1, it produces an advantageous effect that “it allows a compact design by integrally-mounting the leg member of the invention in the middle portion, and thereby solves the problems; therefore, it can be compatible with conventional casters” (Advantageous effect of the invention in the specification) or similar advantageous effects.
Furthermore, Patent document 2 describes a second example of a vehicle with two parallel wheels in related art. Patent document 2 describes a wheelchair capable of safely and reliably overriding a step. The wheelchair described in Patent document 2 is characterized by that “in the wheelchair comprising a frame having a seat for a rider and equipped with pairs of front wheels and rear wheels, the wheelchair further comprises a footrest having substantially the same width as the wheelchair, the footrest comprising an upper side where the rider puts its feet and an under side contact surface for contacting the corner of a step, the footrest being located between the front wheels and mounted on the frame through a substantially horizontal pivot, the pivot being perpendicular to the moving direction of the wheel chair, and the footrest being rotatable between the regular position where the contact surface is located in front of the front wheels and retract position where the contact surface intersects with the lower periphery of the front wheel”.
In accordance with the wheel chair having such structure of Patent document 2, it produces an advantageous effect that “when the rider drives the rear wheels after the wheel chair moves forward until the contact surface contacts the corner of a step with the footrest being in the regular position, the footrest is retract to the retract position against a bias mean so that the front wheels can run on the step, and thereby the rider can further drive the rear wheels to move the entire wheel chair above and past the step” (Paragraph [0021] in the specification) or similar advantageous effects.
Furthermore, Patent document 3 describes a third example of a vehicle with two parallel wheels of in related art. Patent document 3 describes a free wheel used as a front wheel capable of smoothly clearing a step and lessening the impact caused by the contact with the step. The step clearing wheel described in Patent document 3 is characterized by that “in a free wheel for a self-propelled vehicle or a carriage, a main wheel is arranged eccentrically by an pivot, and support wheels are arranged ahead of the main wheel on the left and right sides; and the step clearing wheel further comprises a cushion device for lessening the impact caused by the contact with the step”.
In accordance with the step clearing wheel of Patent document 3, it produces an advantageous effect that “when the wheel encounters a step, the support wheels first hit the step, position the main wheel body perpendicularly to the step, and then runs on the step; at this point, the main wheel (2) does not sustain the weight, and the main wheel (2) starts to run on the step from the position where the support wheels (3) is raised from the ground; furthermore, it clears the step using the cushion to lessen the impact occurring from the contact to the step; and therefore, when it is used for a wheelchair, is can clear a step by forward propulsion force alone regardless of the size of a step” (Paragraph [0006] in the specification) or similar advantageous effects.    [Patent document 1] Japanese unexamined patent application publication No. 2002-2206    [Patent document 2] Japanese unexamined patent application publication No. 2005-296606    [Patent document 3] Japanese utility model registration No. 3105824
However, the step clearing wheel of Patent document 1 is used as a component for transfer equipment. In other word, it is used as a support wheel for clearing a step in a vehicle such as a wheelchair and a baggage transfer vehicle which has 3 or 4 wheels. Furthermore, the wheelchair of Patent document 2 relates to a four-wheel vehicle (wheelchair) which has two front wheels of a smaller diameter and two rear wheels of a larger diameter, and particularly, relates to a wheelchair having a step clearing means capable of sufficiently sustaining the load during the step clearing action. Furthermore, Patent document 3 describes a front free wheel which enables a wheelchair or a handcart to smoothly clear a step and lessens the impact by a cushion.
Since there is no situation where all wheels run on a step at the same time in such three or four wheel vehicle, it can relatively easily clear the step by dividing the vehicle weight between wheels over the step and wheels under the step and producing the driving force at the driving wheels depending on the weight ratio. However, in the case of vehicle with two parallel wheels which runs with parallel-arranged two wheels, all wheels may run on a step simultaneously when the vehicle encounters the step. In such case, it requires enough driving force to run on the step while sustaining the full weight of the vehicle. Therefore, there has been a problem that it requires torque three or four times as large as that needed when driving on a flat place, and thereby requires a driving source capable of producing large driving force.
At this point, it is conceivable that a step run-on mechanism such as those described in Patent document 1-3 may be applied to a vehicle with two parallel wheels. However, there is a problem that when such kind of step run-on assist mechanisms is used in a two wheeled vehicle, the driving wheel will be raised from the ground and spin freely without pushing the ground during a step run-on process. Furthermore, in the case of the step run-on mechanism of Patent document 3, a gas spring is used between the distal end of the arm and the wheel mounting fixture. However, the gas spring is used just for lessening the impact occurring from the contact and is not used for any other purposes.